This disclosure relates generally to methods and apparatus for drilling offshore oil and gas wells. More specifically, this disclosure relates to methods and apparatus for supporting dual gradient drilling operations. Still more specifically, this disclosure relates to methods and apparatus to remove drilling fluids from a riser so as to reduce bottom hole pressure in a wellbore.
Offshore drilling rigs utilize drilling risers as the conduit between the drilling equipment at the surface and drilling equipment mounted on the seafloor. The drilling riser is a tubular conduit that serves as an extension of the wellbore from the equipment on the wellhead at the seafloor to the floating drilling rig. Conventional drilling risers include a primary tubular conduit and a plurality of smaller, higher pressure auxiliary conduits that are externally mounted to the primary tubular and provide conduits for choke, kill, and auxiliary fluid communication with the subsea blowout preventers.
The drilling string is inserted through the riser and into the wellbore. Drilling fluid is pumped into the wellbore through the drill string and returns to the drilling rig via the riser. The drilling fluid serves to lubricate and cool the drill string and carries cuttings from the formation out of the wellbore. The column of drilling fluid contained in the wellbore and the riser creates a hydrostatic pressure within the wellbore that serves as the primary barrier to the influx of wellbore fluids into the wellbore.
The hydrostatic pressure within the wellbore must be kept within a specific range of pressures. If the hydrostatic pressure is too low, formation fluids can enter the wellbore and cause a blow out of the well. If hydrostatic pressure is too high, the rate of penetration of the drill string can be negatively impacted and/or the drilling fluids could cause fractures in the formation. In conventional drilling operations, hydrostatic pressure is regulated by controlling the density of the drilling fluid.
In certain offshore drilling applications, the water depth in which the well is located and the depth of the well being drilled may require a significant height of the column of drilling fluid. As the height of the column of drilling fluid increases, management of pressure within the wellbore by regulating drilling fluid density can become very difficult and limit the fluids that can be used for drilling. Dual gradient drilling operations have been developed to address these types of situations.
Dual gradient drilling operations seek to remove, or reduce the density of, a portion of the column of drilling fluid. This is often accomplished by removing a portion of the drilling fluid from the riser, or injecting a low density fluid into the riser, at a location between the wellhead and the drilling rig. This has the effect of decreasing the height of the column of drilling fluid, thus reducing the hydrostatic pressure acting on the wellbore and allowing higher density drilling fluids to be used in the well.
One problem facing dual gradient drilling systems is the additional equipment necessary to handle the drilling fluid and interface with both the riser and the drilling rig. Many dual gradient drilling systems rely on subsea pumps and other subsea fluid handling equipment. Providing power and control inputs to subsea equipment adds to the complexity of the system and the increases demands on the rig-based equipment and control systems. Many existing rigs are unable to support dual gradient drilling activities without major refit and/or addition of extra equipment.
Thus, there is a continuing need in the art for methods and apparatus for supporting dual gradient drilling operations with existing, or easily added, rig equipment that overcome these and other limitations of the prior art.